This invention relates generally to fuses, and more particularly to a fuse arrangement for thick film circuits.
Thick film resistors are well known and are commonly formed by applying a paste of a resistive material onto an insulating substrate; the applied resistive material is then dried and hardened by the application of heat. The magnitude of the resistance of such a resistor depends upon the thickness of the resistive material, the composition of the material, and upon the extent of the area of the substrate covered with the resistive material.
One application of thick film resistors is as battery feed resistors in battery feed circuits for telephones. In such an application a source of power (e.g. a battery) is connected to a balanced two-wire subscriber telephone line (sometimes referred to as a loop) via two battery feed resistors in order to provide current to the line. The two battery feed resistors should be closely matched to each other in order to maintain the balance of the line, and are preferably small in physical size. The battery feed resistors should also be able to limit current on extremely short loops to safe values; at the same time they must be capable of carrying large currents that may arise from fault conditions (e.g. current surges due to lightning).
The following U.S. patents depict various thick film resistors and attention is directed to them: U.S. Pat. No. 3,573,703 dated Apr. 6, 1971 by D. P. Burks et al; U.S. Pat. No. 3,787,965 dated Jan. 29, 1974 by T. F. Cocca et al; and U.S. Pat. No. 4,293,839 dated Oct. 6, 1981 by E. Asada et al.
It can be seen that the above-mentioned patents are silent on the issue of fusing. U.S. Pat. No. 4,197,521 dated Apr. 8, 1980 by R. M. Rovnyak is directed to the issue of fusing a resistor used in a telephone line circuit. In fact, that patent is directed to making the resistor itself be a fusible element, and does not describe a separate and discrete fuse.